Just another addition: After an extended research over the different guitar and woodworkers' forums it seems to me that a heat gun is definitely the first choice for removing a polyester finish, but for polyurethane a scraper and sandpaper are the best option.
How can you tell the difference? The polyester finish bubbles with moderate heat and peels off in large flakes easily. It is usually slightly orange-coloured, so it alters the natural colour of the wood.
The polyurethane finish is crystal clear and without any colour when fresh, but easily gets opaque (white) when it gets older, especially in a thicker layer. It doesn't peel off with heat, but burns into the wood if it is exposed to heat.
Music to hear, why hear'st thou music sadly?
Sweets with sweets war not, joy delights in joy...
William Shakespeare, Sonnet 8